Overall design of wireless sensor network nodes[Copy link]
Wireless Sensor Networks (WSNs) are composed of a large number of cheap micro sensor nodes deployed in the monitoring area. Through wireless communication, they form a multi-hop self-organizing network system, which can monitor, sense and collect various information of the monitored objects in the network distribution area in real time, and process them to complete data collection and monitoring tasks. WSNs integrate technologies such as sensors, embedded computing, wireless communications, and distributed information processing. They have the characteristics of fast construction, self-configuration, self-adjusting topology, multi-hop routing, high density, variable number of nodes, no unified address, and wireless communication. They are particularly suitable for real-time information monitoring under conditions such as large ranges, remote distances, and dangerous environments. They can be widely used in various fields such as military, transportation, environmental monitoring and forecasting, health care, and space exploration. 2 Overall design of nodes and device selection 2.1 Overall design of nodes The number of WSNs micro nodes is relatively large, and it is difficult to replace and maintain them. The nodes are required to be low-cost and have as long working time as possible; in terms of function, there should be no dedicated router nodes in WSNs, and each node is both a terminal node and a router node. The nodes are connected by mobile ad hoc networks and communicate by multi-hop routing mechanisms. Therefore, on a single node, the hardware must be low-energy and use wireless transmission; on the other hand, the software must support multi-hop routing protocols. Based on these basic ideas, a WSNs micro-node is designed with a high-end 8-bit AVR microcontroller ATmega128L as the core, combined with peripheral sensors and a 2.4 GHz wireless transceiver module CC2420. The size of these two devices is very small, and with the peripheral circuits, the overall size is also very small, which is very suitable for use as a component of WSNs nodes.
Figure 1 shows the structure of a WSNs micro-node. It consists of four parts: a data acquisition unit, a data processing unit, a data transmission unit, and a power management unit. The data acquisition unit is responsible for the collection and data conversion of information in the monitoring area. The design includes combustible gas sensors and humidity sensors; the data processing unit is responsible for controlling the processing operations, routing protocols, synchronous positioning, power consumption management, task management, etc. of the entire node; the data transmission unit is responsible for wireless communication with other nodes, exchanging control messages and sending and receiving collected data; the power management unit selects the sensors used. The node power supply consists of several AA batteries. In actual industrial applications, micro button batteries are used to further reduce the volume. For the convenience of debugging and scalability, the data acquisition unit can be separated and made into two expandable motherboards that can be connected to each other. 2.2 Processor selection The selection requirements and indicators of the processor are low power consumption, ensuring smooth operation without changing the power supply for a long time, the supply voltage is less than 5 V, and has a faster processing speed and ability. Since the nodes need to be placed in large quantities, the price should also be relatively cheap. AVR microcontroller is selected. Considering the small number of I/O in the circuit, low power consumption, low cost, and suitable for interfacing with wireless devices, after comprehensive comparison, Atmel's ATmega128L is selected. This microcontroller has rich on-chip resources, including 4 timers, 4 KB SRAM, 128KB Flash and 4 KBEEPROM; it has UART, SPI, I2C, and JTAG interfaces, which are convenient for access to wireless devices and sensors; it has 6 power saving modes, which is convenient for low-power design. 2.3 Selection of wireless communication devices CC2420 is a highly integrated industrial RF transceiver that complies with ZigBee technology. Its MAC layer and PHY layer protocols comply with the 802.15.4 specification and operate in the 2.4 GHz frequency band. The device requires very few external components to ensure the effectiveness and reliability of short-distance communication. The data transmission unit module supports a data transmission rate of up to 250 Kb/s, which can realize multi-point to multi-point rapid networking. The system is small in size, low in cost, low in power consumption, suitable for long-term battery power supply, and has the characteristics of hardware encryption, security and reliability, flexible networking, and strong anti-destruction. 2.4 Sensor Selection Since WSNs are used for underground mine safety monitoring, it is often necessary to detect the concentration of combustible gas in the mine (to prevent excessive gas concentration) and air humidity, so sensors that measure gas concentration and humidity should be selected. 2.4.1 HIH-4000 Series Humidity Sensors The HIH-4000 Series Humidity Sensors provide instrument-quality relative humidity (RH) sensing performance in a low-cost, solderable, single-in-line package (SIP). Available in two-lead spaced-apart packages, the RH sensor is a thermoset capacitive sensing element with internal signal processing. The multi-layer construction of the sensor provides optimal resistance to the adverse elements of the application environment, such as moisture, dust, dirt, oils, and chemicals commonly found in the environment, making it suitable for use in underground mine environments. 2.4.2 MR511 hot-wire semiconductor gas sensor The MR511 gas sensor uses the principle that the gas adsorbed on the surface of the metal oxide semiconductor produces changes in thermal conductivity and electrical conductivity, and measures the gas concentration by the change in the resistance value of the platinum coil. The MR511 consists of a detection element and a compensation element paired to form the two arms of the bridge. When encountering flammable gas, the resistance of the detection element decreases, and the output voltage of the bridge changes. The voltage change increases proportionally with the increase in gas concentration, and the compensation element has a temperature compensation effect. In addition to its high sensitivity, short response recovery time, and good stability, the MR511 also has the advantages of low power consumption and strong resistance to environmental temperature and humidity interference. The energy-saving requirements of WSNs and the harsh temperature and humidity environment underground can be met by MR5111.
3 WSNs node design 3.1 Data acquisition unit Considering the energy saving of wireless sensor network nodes and the harsh temperature and humidity environment underground, in order to facilitate data acquisition, the system design adopts HIH-4000-01 humidity sensor and MR511 hot wire semiconductor gas sensor. Figure 2 and Figure 3 give their circuit design diagrams respectively. 3.2 Data processing unit The peripheral circuit design of ATmega128L is simple. When designing, pay attention to the power supply of the digital circuit and add multiple capacitor filters. The working clock source of ATmega128L can be selected from external crystal oscillator, external RC oscillator, internal RC oscillator, external clock source, etc. The selection of the working clock source is designed by the internal fuse bit of ATmega128L. The fuse bit can be set by JTAG programming, ISP programming, etc. ATmega128L uses two external crystal oscillators of 7.3728 MHz and 32.768 kHz. The former is used as the working clock and the latter is used as the real-time clock source. 3.3 Data Transmission Unit 3.3.1 CC2420 peripheral circuit design Figure 4 shows the peripheral circuit of the data transmission unit. CC2420 requires only a few peripheral components. Its peripheral circuit includes three parts: crystal oscillator clock circuit, RF input/output matching circuit and microcontroller interface circuit. The RF input/output matching circuit is mainly used to match the input and output impedance of the device to 50 Ω, and at the same time provide DC bias for the PA and LNA inside the device. The RF input/output is high impedance and has differences. The most suitable load for the RF end is 115+j180 Ω. C61, C62, C71, C81, and L61 form an unbalanced transformer. L62 and L81 match the RF input and output to 50 Ω; L61 and L62 provide DC bias for the power amplifier and low noise amplifier at the same time. The internal T/R switch is used to switch the low noise amplifier/power amplifier. The R451 bias resistor is a precision resistor for the current reference generator. The local oscillator signal of CC2420 can be provided by an external active crystal or by an internal circuit. If it is provided by an internal circuit, an external crystal oscillator and two load capacitors are required. The size of the capacitor depends on the frequency of the crystal and the input capacitance. When a 16 MHz crystal is used, the capacitance value is about 22 pF. C381 and C391 are the load capacitors of the external crystal oscillator. The on-chip voltage regulator provides all internal 1.8 V power supplies. C42 is the load capacitor of the voltage regulator, which is used to stabilize the regulator. Power supply decoupling must be used to obtain the best performance. It is very important to use the right size decoupling capacitors and power filters in the application. CC2420 can set the device's operating mode through the 4-wire SPI bus (SI, SO, SCLK, CSn), and realize reading, writing cache data, reading/writing status registers, etc. The transmit/receive buffer can be set by controlling the state of the FIFO and FIFOP pin interfaces. 3.3.2 Configure IEEE 802.15.4 working mode CC2420 provides hardware support for the data frame format of IEEE 802.15.4. The frame format of its MAC layer is: header frame + data frame + check frame; the frame format of the PHY layer is: synchronization frame + PHY header frame + MAC frame. The length of the frame header sequence can be changed by setting the register, and a 16-bit CRC check is used to improve the reliability of data transmission. The data frame sent or received is sent to the 128-byte buffer area in the RAM for corresponding frame packaging and unpacking operations. Table 1 shows the pin functions of the four-wire serial SPI interface of CC2420. It is the basis for designing microcontroller circuits, and giving full play to these functions is the premise for designing wireless communication products. 3.3.3 CC2420 and MCU interface circuit design
[attach]416133 [/attach]
Figure 5 shows the interface circuit between CC2420 and ATmega128L MCU. CC2420 is configured with a SPI-compatible serial interface through a simple four-wire (SI, SO, SCLK, CSn), and CC2420 is controlled at this time. The SPI interface of ATmega128L works in master mode, which is the controller of SPI data transmission; CC2420 is set to slave mode. When the SPI interface of ATmega128L is set to master mode, its hardware circuit will not automatically control the SS pin. Therefore, when SH communicates, the SPI interface should be initialized. It is controlled by the program to pull SS to a low level. After that, when the data is written into the host's SPI data register, the host interface will automatically start the clock generator. Under the control of the hardware circuit, the shift transmission is carried out, and the data is moved out of ATmega128L through MOSI, and at the same time, the data is moved in from CC2420 through MISO. When all 8 bits of data are moved out, the two registers have realized a data exchange. 4 Conclusion Through the selection of sensor elements, data processing modules, data transmission modules and power supplies in wireless sensor network nodes, a hardware solution based on CC2420 and ATmega128L is designed. The peripheral circuits of CC2420 and ATmega128L and the interface circuit between the two are designed using this solution. In addition, the interface circuit between the sensor and the microcontroller is also designed. Through experimental verification, the designed hardware node basically meets the project requirements. After debugging, it can correctly and truly collect data through sensors, and realize the communication and data transmission between two wireless nodes (two circuit boards. Powered by AA batteries) at a distance of about 30 m, and reflect it to the terminal device. 3 CC2420 and MCU interface circuit design
[attach]416133 [/attach]
Figure 5 shows the interface circuit between CC2420 and ATmega128L MCU. CC2420 is configured with a SPI-compatible serial interface through a simple four-wire (SI, SO, SCLK, CSn), and CC2420 is controlled at this time. The SPI interface of ATmega128L works in master mode, which is the controller of SPI data transmission; CC2420 is set to slave mode. When the SPI interface of ATmega128L is set to master mode, its hardware circuit will not automatically control the SS pin. Therefore, when SH communicates, the SPI interface should be initialized. It is controlled by the program to pull SS to a low level. After that, when the data is written into the host's SPI data register, the host interface will automatically start the clock generator. Under the control of the hardware circuit, the shift transmission is carried out, and the data is moved out of ATmega128L through MOSI, and at the same time, the data is moved in from CC2420 through MISO. When all 8 bits of data are moved out, the two registers have realized a data exchange. 4 Conclusion Through the selection of sensor elements, data processing modules, data transmission modules and power supplies in wireless sensor network nodes, a hardware solution based on CC2420 and ATmega128L is designed. The peripheral circuits of CC2420 and ATmega128L and the interface circuit between the two are designed using this solution. In addition, the interface circuit between the sensor and the microcontroller is also designed. Through experimental verification, the designed hardware node basically meets the project requirements. After debugging, it can correctly and truly collect data through sensors, and realize the communication and data transmission between two wireless nodes (two circuit boards. Powered by AA batteries) at a distance of about 30 m, and reflect it to the terminal device. 3 CC2420 and MCU interface circuit design
[attach]416133 [/attach]
Figure 5 shows the interface circuit between CC2420 and ATmega128L MCU. CC2420 is configured with a SPI-compatible serial interface through a simple four-wire (SI, SO, SCLK, CSn), and CC2420 is controlled at this time. The SPI interface of ATmega128L works in master mode, which is the controller of SPI data transmission; CC2420 is set to slave mode. When the SPI interface of ATmega128L is set to master mode, its hardware circuit will not automatically control the SS pin. Therefore, when SH communicates, the SPI interface should be initialized. It is controlled by the program to pull SS to a low level. After that, when the data is written into the host's SPI data register, the host interface will automatically start the clock generator. Under the control of the hardware circuit, the shift transmission is carried out, and the data is moved out of ATmega128L through MOSI, and at the same time, the data is moved in from CC2420 through MISO. When all 8 bits of data are moved out, the two registers have realized a data exchange. 4 Conclusion Through the selection of sensor elements, data processing modules, data transmission modules and power supplies in wireless sensor network nodes, a hardware solution based on CC2420 and ATmega128L is designed. The peripheral circuits of CC2420 and ATmega128L and the interface circuit between the two are designed using this solution. In addition, the interface circuit between the sensor and the microcontroller is also designed. Through experimental verification, the designed hardware node basically meets the project requirements. After debugging, it can correctly and truly collect data through sensors, and realize the communication and data transmission between two wireless nodes (two circuit boards. Powered by AA batteries) at a distance of about 30 m, and reflect it to the terminal device.
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